Method of forming a p-type group II-VI semiconductor crystal layer on a substrate

ABSTRACT

A method of depositing a p-type magnesium-, cadmium- and/or zinc-oxide-based II-VI Group compound semiconductor crystal layer over a substrate by a metalorganic chemical vapor deposition technique. A reaction gas is supplied to a surface of a heated substrate in a direction parallel or oblique to the substrate. The p-type magnesium-, cadmium- and/or zinc-oxide-based II-VI Group compound semiconductor crystal layer is grown on the heated substrate, while introducing a pressing gas substantially in a vertical direction toward the substrate to press the reaction gas against the entire surface of the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a U.S. divisional application filed pursuantto Title 35, United States Code §§100 et seq. and 37 C.F.R. Section1.53(b) claiming priority under Title 35, United States Code §119(e) toU.S. provisional application No. 60/391,507 filed Jun. 24, 2002 andnonprovisional application Ser. No. 10/422,568 filed Apr. 23, 2003naming as inventors Jeffrey E. Nause, Joseph Owen Maciejewski, andVincente Munne as inventors, which applications are incorporated hereinby reference. Both the subject divisional application, nonprovisionalpatent application, and its provisional application have been or areunder obligation to be assigned to the same entity.

STATEMENT OF GOVERNMENT RIGHTS IN THE INVENTION

This-invention was made pursuant to a Small Business Innovative Researchproject funded by the U.S. Government as represented by the Office ofNaval Research under Contract No. N00014-00-C-0362. The U.S. Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to forming compound semiconductor layers usingtechniques such as metal-oxide chemical vapor deposition (MOCVD) ormetal-oxide vapor phase epitaxy (MOVPE). More particularly, thisinvention relates to methods for forming magnesium, cadmium, and/or zincoxide crystalline semiconductor layers useful for making electrical andelectro-optical devices such as light emitting diodes (LEDs), laserdiodes (LDs), field effect transistors (FETs), and photodetectors.

2. Description of the Related Art

For some time there has been interest in producing II-VI compound wideband gap semiconductors to produce green/blue LEDs, LDs and otherelectrical devices. Historically, attempts to produce these devices havecentered around zinc selenide (ZnSe) or gallium nitride (GaN) basedtechnologies. However, these approaches have not been entirelysatisfactory due to the short lifetime of light emission that resultsfrom defects, and defect migration in these devices.

Recently, because ZnO has a wide direct bandgap of 3.3 electron-Volts(eV) at room temperature and provides a strong emission source ofultraviolet light, ZnO thin films on suitable supporting substrates havebeen proposed as new materials for LEDs and LDs. Undoped, as well asdoped, ZnO films generally show n-type conduction. Impurities such asaluminum and gallium in ZnO films have been studied by Hiramatsu et al.who report activity as n-type donors (Transparent Conduction Zinc OxideThin Films Prepared by XeCl Excimer Laser Ablation, J. Vac. Sci.Technol. A 16(2), March/April 1998). Although n-type ZnO films have beenavailable for some time, the growth of p-type ZnO films necessary tobuild many electrical devices requiring p-n junctions has been muchslower in developing.

Minegishi et al. (Growth of P-Type ZnO Films by Chemical VaporDeposition, Jpn. J. Appl. Phys. Vol. 36 Pt. 2, No. 11A (1997)) recentlyreported on the growth of nitrogen doped ZnO films by chemical vapordeposition and on the p-type conduction of ZnO films at roomtemperature. Minegishi et al. disclose the growth of p-type ZnO films ona sapphire substrate by the simultaneous addition of NH₃ in carrierhydrogen and excess Zn in source ZnO powder. When a Zn/ZnO ratio of 10mol % was used, secondary ion mass spectrometry (SIMS) confirmed theincorporation of nitrogen into the ZnO film, although the nitrogenconcentration was not precisely confirmed. Although the films preparedby Minegishi et al. using a Zn/ZnO ratio of 10 mol % appear toincorporate a small amount of nitrogen into the ZnO film and convert theconduction to p-type, the resistivity of these films is too high forapplication in devices such as LEDs or LDs. Also, Minegishi et al.report that the carrier density for the holes is 1.5.×10¹⁶ holes/cm³,which is considered to be too low for use in commercial light emittingdiodes or laser diodes.

Park et al. in U.S. Pat. No. 5,574,296 disclose a method of producingthin films on substrates by doping IIB-VIA semiconductors with group VAfree radicals for use in electromagnetic radiation transducers.Specifically, Park et al. describe ZnSe epitaxial thin films doped withnitrogen or oxygen wherein ZnSe thin layers are grown on a GaAssubstrate by molecular beam epitaxy. The doping of nitrogen or oxygen isaccomplished through the use of free radical source which isincorporated into the molecular beam epitaxy system. Using nitrogen asthe p-type dopant, net acceptor densities up to 4.9×10¹⁷ acceptors/cm³and resistivities less than 15 ohm-cm were measured in the ZeSe film.However, the net acceptor density is too low and the resistivity is toohigh for use in commercial devices such as LEDs, LDs, and FETs.

White et al in U.S. Pat. No. 6,291,085 disclose a method for producingZnO films containing p-type dopants, in which the p-type dopant isarsenic and the substrate is gallium arsenide (GaAs). The method ofpreparation of the film is laser ablation. However, the crystal qualityof the films prepared by such a process is inferior and not suitable fordevice applications.

Although some progress has recently been made in the fabrication ofp-type doped ZnO films which can be utilized in the formation of p-njunctions, a need still exists in the industry for ZnO films whichcontain higher net acceptor concentrations and possess lower resistivityvalues.

SUMMARY OF THE INVENTION

The invented method described herein overcomes the disadvantages notedabove with respect to previous techniques for making p-type zinc oxidelayers. This method can be used to make relatively high-quality lightemitting diodes (LEDs), laser diodes (LDs), field effect transistors(FETs), and photodetectors, and other electrical, electro-optic, oropto-electrical devices.

Broadly stated, a method in accordance with the invention comprisesforming a magnesium, cadmium, and/or zinc oxide(Mg_(1−x−y)Cd_(x)Zn_(y)O; 0≦x<1, 0<y≦1, and x+y=0.1 to 1) II-VI Groupcompound semiconductor crystal layer with a p-type dopant uniformlyincorporated into the layer, with said layer having relatively highcrystal quality. The method can be implemented using metalorganicchemical vapor deposition (MOCVD) or metalorganic vapor phase epitaxy(MOVPE) techniques.

In one embodiment, the method for growing a p-type ZnO-based film,optionally with magnesium or cadmium, on a ZnO substrate can comprisecleaning a ZnO substrate. The cleaning can be performed to ensure that aZnO film can be formed on the ZnO substrate with a reduced number ofdefects, and will also properly adhere to the substrate. The method canfurther comprise heating the substrate. The heating of the substratecauses the reaction gases containing magnesium, cadmium, and/or zinc andoxygen, and p-type dopant atoms, to bond and integrate with thesubstrate to form a relatively crystalline p-type semiconductor layer.The substrate can be heated to a temperature between 250 degrees celsius(C) and about 650 degrees C. The method can further comprise supplyingreaction gases into a chamber containing the ZnO substrate. The reactiongases contain zinc and oxygen elements and p-type dopant atoms forforming the p-type ZnO layer. The p-type dopant gas can include one ormore atomic elements from Groups IA, IB, VA and/or VB of the periodictable of the elements. For example, such element can include magnesium,cadmium, copper, arsenic, phosphorus, and others. The reaction gases canbe admitted into the chamber at flow rates ranging from ten (10) tofive-thousand (5000) standard cubic centimeters per minute (sccm). Themethod can comprise entraining one or more of the reaction gases into aflow of inert gas for delivery to the substrate's surface.

By supplying the reaction gases into the chamber containing the heatedsubstrate, a p-type ZnO-based crystalline semiconductor layer can begrown with the method. The reaction forming the layer can be permittedto continue for sufficient time to produce a ZnO-based layer of a targetthickness permitting formation of electrical or electro-optical devicestherein.

The p-type ZnO-based layer produced by the disclosed method can be usedin LEDs, LDs, FETs, and photodetectors, in which both n-type and p-typematerials are required, as a substrate material for lattice matching toother materials in such devices, and/or as a layer for attachingelectrical leads, among other possible uses.

Additional objects and advantages of the invention are set forth in thedescription which follows. The objects and advantages of the inventionmay be realized and obtained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutea part of the specification, illustrate presently preferred embodimentsof the invention, and together with the general description given aboveand the detailed description of the preferred embodiments given below,serve to explain the principles of the invention.

FIG. 1 shows an apparatus of the invention, which can be suitably usedfor the practice of the method of the present invention.

FIG. 2 is a flowchart of a method for producing aMg_(1−x−y)Cd_(x)Zn_(y)O II-VI Group compound semiconductor crystal layeron a substrate in accordance with the invention.

The invention is now described with reference to the accompanyingdrawings which constitute a part of this disclosure. In the drawings,like numerals are used to refer to like elements throughout the severalviews.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a method of depositing a ZnO-basedII-VI Group compound semiconductor crystal layer on a substrate by themetalorganic chemical vapor deposition or metalorganic vapor phaseepitaxy technique. ZnO-based II-VI Group compound semiconductors includezinc oxide (ZnO), magnesium zinc oxide (MgZnO), cadmium zinc oxide(CdZnO), and magnesium cadmium zinc oxide (MgCdZnO). Thesesemiconductors may be represented by the formula:Mg _(1−x−y) Cd _(x) Zn _(y) O, in which 0≦x<1, 0<y≦1, and x+y=0.1 to 1.

In the present invention, a reaction gas comprising a firstorganometallic gas containing zinc, and optionally also magnesium and/orcadmium, and a second gas containing oxygen, are supplied to the surfaceof the heated substrate surface. The substrate can comprise a bulk ZnOcrystal chemically matched to the ZnO layer to be formed. A third dopantgas is also supplied to the heated surface to produce p-typeconductivity by introducing dopant atoms into the ZnO layer as it grows.The ZnO-based II-VI Group compound semiconductor crystal grows on theheated substrate surface through the reaction of the first and secondgas, and the p-type dopant is uniformly incorporated into the lattice ofthe crystal as it grows.

The first gas can be at least an organozinc compound such asdiethylzinc, dimethylzinc or a mixture thereof. The first gas canfurther contain an organic compound of a Group III element, other thanorganogallium compounds. Examples of such an organic compound includesan organomagnesium compound such as bis(cyclopentadienyl)magnesium,bis(methylcyclopentadienyl)magnesium or a mixture thereof, or anorganocadmium compound such as dimethylcadmium.

The second gas is oxygen which can react with the first gas to produce alayer of Mg_(1−x−y)Cd_(x)Zn_(y)O compound on the substrate.

The third gas contains a gaseous p-type dopant source, such asmetalorganic or other precursors from Groups IA, IB, VA or VB from theperiod table of the elements. In the preferred embodiment, p-type dopantsources include bis(tetramethylheptanedianol)copper, arsine, phosphine,or tertiarybutylphosphine to introduce p-type dopant atoms copper,arsenic, and phosphorus, respectively, into the Mg_(1−x−y)Cd_(x)Zn_(y)Olayer.

FIG. 1 schematically shows an apparatus 20 that can be used to performthe method of the present invention. As shown in FIG. 1, the apparatus 1has a reaction chamber 2 made of, for example, stainless steel. Acarrier plate 3 and susceptor 4 are arranged in the chamber 2 to place asubstrate 5 thereon substantially horizontally. The substrate(s) 5 isloaded/unloaded through a load/unload port 6 arranged in the chamber 2,as is well known in the art. The susceptor 4 is a round column having adiameter of, for example, thirty (30) to one-hundred-fifty (150)millimeters (mm) and a height of, for example, ten (10) to thirty (30)mm. The carrier plate 3 and susceptor 4 are made of a highheat-resistant material which does not contaminate the gases in thechamber 2 upon heating. Such a material includes carbon surface-coatedwith silicon carbide.

A shaft 7 is fixed to the center of the lower surface of the susceptor4, and air-tightly extends outside the chamber 2. The shaft 7 can beconnected to and rotated by a drive unit 8, such as an electric motor orservo motor, to rotate the susceptor 4, and hence the carrier plate 3and substrate 5, during the growth of an Mg_(1−x−y)Cd_(x)Zn_(y)O layer 9on the respective substrate 5. The drive unit 8 is connected to therotation speed controller 9 that regulates the rotation speed impartedby the drive unit 8 to the shaft 7 to maintain a target rotation speed.The target rotation speed can be constant, or alternatively, can betime-varying according to a predetermined rotation-speed-versus-timeprofile. In some implementations of the method, it can be desirable, forexample, to set the rotation speed at a relatively slow rate at thebeginning of the process of forming the Mg_(1−x−y)Cd_(x)Zn_(y)O layer(s)9, speeding up to be relatively fast toward the end of the process offorming the layer. The controller 10 can be a computer that senses therotation speed of the shaft 7. For example, the rotation speed can besensed by a tachometer internal to the drive unit 8, which generates anelectric signal indicating the rotation speed on conductive line 11. Therotation speed controller 10 senses the rotation speed, and subtracts itfrom the target rotation speed internally to the controller 10, togenerate an error signal. As previously mentioned, the target rotationspeed can be constant in which case it is time-invariant over the entireprocess. Alternatively, the target rotation speed corresponding to thetime elapsed from the start of the timed process can be read from atarget rotation speed profile, in which case the target speed for theelapsed time from commencement of the process is read by the controller10 from its memory, and subtracted from the rotation speed signal, togenerate the error signal. The rotation speed controller 10 uses theerror signal to adjust the magnitude of the drive current to eitherspeed, slow, or maintain the rotation speed, based on the error signal.The controller 10 supplies the adjusted drive current to the conductiveline 12 to the drive unit 8 to drive the shaft 7 to rotate, thereby alsorotating the susceptor 4 and carrier plate 3, to rotate the substrate(s)5.

A heater 13 is arranged to heat the susceptor 4, and hence the carrierplate 3 and substrate 5, to a temperature suitable to grow the desiredZnO-based II-VI Group compound semiconductor crystal layer on thesubstrate, e.g., about 400 degrees C. or more. In FIG. 1, such theheater 13 is provided close to, but away from, the lower surface of thesusceptor 4. The heater 13 can be an electrically-resistive element thatis controlled to heat the susceptor 4 to the required temperature by atemperature controller 14 and a temperature sensor 15. The temperaturesensor 15 can be incorporated in the susceptor 4. The temperature sensor15 can be a thermistor, for example. The temperature sensor 15 generatesan electric signal proportional to the temperature it senses. The sensor15 is connected via electrically-conductive lines 16, 17 such as metalwires, and slip ring 18. Because the shaft 7 rotates relative to thetemperature controller 14, slip ring 18 is connected between the wire 16from temperature sensor 15 and a conductive wire 17 to temperaturecontroller 14. Alternatively, an optical temperature sensor 19 can bearranged to view the substrate(s) 5 through transparent window 20air-tightly sealed in the wall of chamber 2. The optical temperaturesensor 15 generates the sensed temperature signal on line 21 that issupplied to the temperature controller 14. The temperature controller 14can include an on/off control or supply current control, for example,that generates electric current based on the signal from the temperaturesensor 15 or 17. The temperature controller 14 is connected to supplythe electric current it generates to the heating element 13 viaelectrically-conductive line 22 which extends through bushing 23 in thewall of chamber 2. A user can set a target temperature with thetemperature controller 14. Alternatively, if a user desires to vary thetemperature of the substrate(s) 5 during the layer growth process, theuser can set a target-temperature-versus-time profile to control thetemperature of the substrate as a function of time during the layergrowth process. If the temperature sensed by the sensor 15 or 17 isbelow the target temperature, the temperature controller 14 supplieselectric current on the line 22 to the heating element 13 to increasethe temperature of substrate(s) 5. Conversely, if the sensed temperatureis at or above the target temperature, the temperature controller 14does not generate electric current to permit the substrate(s) 5 to coolto the target temperature. The temperature sensor 15 or 17 andcontroller 14 thus function to maintain the temperature of thesubstrate(s) 5 and the respective layer(s) 9 growing thereon, at thetarget temperature during the growth process.

Injection tubes 24, 25, 26 extend through a wall of chamber 2, and areair-tightly sealed thereto. The injection tubes 24, 25, 26 can bearranged perpendicularly or transversely, to the substrate(s) 5positioned on the carrier plate 5. Zinc-, cadmium-, andmagnesium-containing reaction gases are blown through respectiveinjection tubes 24, 25, 26 perpendicularly to the substrate surface,together with a carrier gas such as argon gas.—More specifically, theinjection tubes 24, 25, 26 can be connected to respective pressure flowcontrollers 27, 28, 29. The pressure flow controllers 27, 28, 29 are inturn connected to bubblers 30, 31, 32 containing respective liquidzinc-, cadmium-, and magnesium-organic compounds 33, 34, 35. Thebubblers 30, 31, 32 are connected to respective mass flow controllers36, 37, 38. The mass flow controllers 36, 37, 38 are connected throughrespective conduits to the tank 39 of carrier gas such as argon. Thepressure and mass flow controllers 27, 28, 29, 36, 37, 38 can beelectrically connected via lines 40 to the flow controller 41. The flowcontroller 41 can be implemented as a computer with an input device suchas dials or keys permitting a user to set the pressure flow rates forcontrollers 27, 28, 29 and the mass flow rates for controllers 36, 37,38. More specifically, the flow controller 41 controls the mass flowcontrollers 36, 37, 38 to regulate the amount of carrier gas permittedto pass into respective liquid zinc-, cadmium-, and magnesium-organiccompounds 33, 34, 35 in the bubblers 27, 28, 29. Through bubbling inliquids 33, 34, 35, the carrier gas flows pick up vaporous zinc-,cadmium-, and magnesium-organic compounds for transport to respectivepressure flow controllers 27, 28, 29. The flow controller 41 controlsthe pressure flow controllers 27, 28, 29 via lines 40 to regulate thepressures at which the respective zinc-, cadmium-, andmagnesium-containing gases are introduced into the chamber 2.

To provide oxygen to form the p-type Mg_(1−x−y)Cd_(x)Zn_(y)O layer(s) 9on respective substrate(s) 5, injection tube 48 is air-tightly sealed tothe wall of chamber 2. The injection tube 42 is connected to a mass flowcontroller 43 which in turn is connected to a tank 44 of oxygen gas. Theinjection tube 42 can also be connected to a mass flow controller 45which is in turn connected to tank 46 of carrier gas such as argon. Themass flow controllers 45 and 46 can be manually-set, or alternatively,can be electronically-controlled with the flow controller 41 via lines40. The mass flow controllers 43, 45 can be used to generate a flow ofcarrier gas from tank 44, along with oxygen containing gas from tank 46,enters the chamber 2 through injection tube 42 and is uniformlydistributed in a vertical flow towards the substrate 5 via adistribution plate 47. In FIG. 1, the distribution plate 47 extendshorizontally across the chamber and partitions the upper portion of thechamber from its lower portion. The distribution plate 47 is positionedabove the points of entry of other reaction gases into the chamber 2 topromote chemical reaction of gases at the surface of the substrate(s) 5rather than in the upper part of the chamber away from the substrate(s).The distribution plate 47 has spaced holes defined therein to dispersethe carrier and oxygen containing gases, and is positioned relative tothe substrate(s) 5 so that the flow of carrier and oxygen-containing gasis directed vertically and thus transversely to the surface(s) of thesubstrate(s) 5 to promote the reaction forming theMg_(1−x−y)Cd_(x)Zn_(y)O layer(s) 9 on the respective substrate(s). Thisoxygen-containing gas flow thus presses the reaction gases against thesubstrate(s) 5 where the reaction occurs.

An injection tube 48 is provided, and has an outer surface air-tightlysealed to the wall of chamber 2. The injection tube 48 can be used forseparate injection of dopant or other reaction gases. The injection tube48 extends through the dispersion plate 47 and can have a diffusion head49 defining spaced holes to diffuse carrier and dopant gas receivedthrough tube 48. The diffusion head 43 diffuses and directs the carrierand dopant gas against the surfaces of the substrate(s) 5 to form theMg_(1−x−y)Cd_(x)Zn_(y)O layer(s) 9 on the respective substrate(s) 5. Ifthe dopant is gaseous, such as a nitrogen- or phosphorus-containing gas,the opposite end of the injection tube 42 can be connected to a massflow controller 50 that is in turn connected to tank 51 which containsthe dopant gas. Alternatively, if the dopant is in liquid form, such asa cadmium- or magnesium-containing volatile liquid, then the injectiontube 48 can be connected to a pressure flow controller 52 which is inturn connected to a bubbler 53 containing the dopant-organic compoundliquid 54. The bubbler 53 is connected to mass flow controller 55 whichis in turn connected to tank 56 of carrier gas such as argon. Thecontrollers 50, 52, 55 can be manually-set, or can beelectronically-controller by the flow controller 41 via lines 40. In thecase of use of a gaseous dopant, the flow controller 41 regulates flowthrough the mass flow controller 50 to a regulated level as set by auser. In the case of liquid dopant, the flow controller 41 controls themass flow controller 55 to permit a regulated amount of carrier gas topass through controller 55 to bubble in the liquid dopant compound 54 togenerate dopant gas that passes at a pressure regulated by controller 41via pressure flow controller 52 at a desired pressure through injectiontube 48 and diffusion head 49 into the chamber 2.

It should be noted that the flow controller 41 can be such as to controlthe mass flow and pressure flow controllers 27, 28, 29, 36, 37, 38, 43,45, 50, 52, 55 to produce constant mass and pressure flows, oralternatively, can vary one or more of such flows during the process ofgrowing the p-type Mg_(1−x−y)Cd_(x)Zn_(y)O layer(s) 9 on the substrateaccording to a flow-versus-time profile stored in the memory of thecontroller 41 by a user.

At the lower portion of the chamber 2, an exhaust tube 57 is air-tightlysealed to the chamber 2. Gases inside the chamber 2 can be exhaustedusing an exhaust pump 58. The exhaust pump 58 can be manually-set tomaintain a specified rate of exhausting of gases from the chamber 2.Alternatively, the exhaust pump 58 can be electronically-controlled viaa pump controller 59. More specifically, the pump controller 59 receivesa pressure signal on line 61 from butterfly valve 60 positioned in theexhaust tube 57. The pressure signal on line 61 indicates the pressureof the gases in the chamber 2. If the pressure of the chamber gases isat or below the target pressure set by a user in the pump controller 59,the pump controller 59 generates the pump signal on line 62 to controlthe pump 58 to slow the rate of exhausting of gases from the chamber.Conversely, if the sensed pressure is above the target pressure set inthe pump controller 59, the pump controller 59 generates the pumpcontrol signal to cause the pump 58 to speed the rate of exhausting ofgases form the chamber 2 to lower the gas pressure inside the chamber 2.The gas pressure within chamber 2 can thus be maintained at a regulatedtarget pressure.

In a mass production environment, it can be desirable to provide theapparatus 1 with a main controller 61. The main controller 61 isconnected to the rotation speed controller 10, the temperaturecontroller 14, the flow controller 41, and the pump controller 59, andcan be used to automatically activate or deactivate the such units toexecute the process of forming the Mg_(1−x−y)Cd_(x)Zn_(y)O layer(s) 9 onrespective substrate(s) 5.

A substrate 5 can be formed, for example, from a stoichiometric powderof elements comprising the target crystal composition with a gasoverpressure using an apparatus and method such as that described inU.S. Pat. No. 5,900,060 issued May 4, 1999 to Jeffrey E. Nause et als.,which is incorporated herein by reference. This patent is commonlyassigned to the owner of this application, Cermet, Inc., Atlanta, Ga.For example, a ZnO powder can be used in this apparatus and method toproduce a ZnO crystal with few or virtually no impurities or defects.The nature of this apparatus and method is such as to produce thecrystal from a liquid phase that is contained by a cooler outer solidphase of the same material. After formation, the substrate is cut,polished, and cleaned with a suitable etchant or other chemical agent toproduce a flat, defect- and contaminant-free surface on which theMg_(1−x−y)Cd_(x)Zn_(y)O layer(s) 9 can be formed.

It is generally preferred that the substrate(s) 5 has crystallinestructure with a lattice spacing closely matched to that of theMg_(1−x−y)Cd_(x)Zn_(y)O layer(s) 9 to be formed thereon. This helps tolower the number of defects in the layer(s) 9 that would otherwise becaused by lattice mismatch. Thus, ZnO is most preferred compound for useas the substrate(s) 5 because of its close or exact match with thelattice structure of the Mg_(1−x−y)Cd_(x)Zn_(y)O layer(s) 9 to be formedthereon. However, this does not exclude the possibility of using othersubstrate compositions, such as sapphire (Al₂O₃), silicon carbide (SiC),silicon (Si), gallium arsenide (GaAs), and gallium nitride (GaN).

In operation of the apparatus 1, a substrate(s) 5 such as a zinc oxide(ZnO) substrate(s) is placed on the carrier plate 3, which issubsequently placed on the susceptor 4 through port 6. The susceptor 4is heated with the heater 13 to a temperature of, for example, 250degrees C. to 1050 degrees C. to heat the substrate 5 to thattemperature. The heater 13 can be controlled by temperature controller14 using a sensed temperature signal from either of temperature sensors15, 19. The susceptor 4, and hence the carrier plate 3 and substrate 5,is rotated by driving the shaft 7 with drive unit 8. The drive unit 8controller to rotate the substrate(s) 5 via shaft 7, susceptor 4, andplate 3 at a constant speed or according to a rotation-speed-versus-timeprofile that varies over time. The dopant, reaction, and carrier gasesare supplied in the chamber 2 through respective injection tubes 24, 25,26, 42, and 48 in a direction perpendicular, or at least transverse, tothe substrate 5. The mass and pressure flows of the reaction gases canbe set either manually or by using flow controller 41 to generatesignals supplied to the controllers 27, 28, 29, 36, 37, 38, 43, 45, 50,52, 55, to regulate the flows of dopant, reaction, and carrier gasesinto the chamber 2. Exhaust pump 58 controls the pressure of chambergases to a target pressure level. This can be done by manual setting ofthe exhaust pump 58. Alternatively, the chamber pressure can be sensedusing the butterfly valve 60 to generate a sensed pressure signal whichthe pump controller 59 uses to generate the pump control signal tocontrol the pump 58 to maintain the chamber gas pressure to the targetpressure level. The chamber pressure can be controlled via the exhaustpump 58 to maintain a pressure of five (5) to fifty (50) torrs ofpressure during the layer growth operation. The exhaust pump 58 cancontrol the chamber gas pressure to a constant target pressurethroughout the layer growth operation, or alternatively, can vary thegas pressure in the chamber 2 according to a pressure-versus-timeprofile stored in the memory of the pump controller 59. Spinning of thesusceptor 4 and the attached carrier plate 3 and substrate 5 convertsthe reaction gas flow to one essentially parallel with the surface(s) ofsubstrate(s) 5. The reactants of the reaction gas react with each otherto grow the desired ZnO-based II-VI Group compound semiconductor crystallayer on the entire substrate surface.

In FIG. 2 a method for forming a p-type zinc-oxide-based crystallinesemiconductor layer on a substrate(s) 5 begins in step S1 in which thesubstrate is placed in the chamber 2 through port 6. In step S2 thechamber 2 is sealed by closing port 6. In step S3 the substrate(s) 5 isrotated at speed from one-hundred (100) to one-thousand (1,000) rpm.This can be done by the drive unit 8 under control of the rotation speedcontroller 10. In step S4 the substrate(s) 5 is heated to a temperaturein a range from two-hundred-fifty (250) to six-hundred-fifty (650)degrees C. This can be done with the heater 13. The substratetemperature can be sensed by the temperature sensor 15 or 19 for use bythe temperature controller 14 to control the heater 13 to regulate thesubstrate temperature. In step S5 the zinc-containing reaction gas,optionally with magnesium- and cadmium-containing reaction gases,oxygen-containing gas, dopant gas, and any carrier gas used to transportthe reaction and dopant gases, are supplied to the substrate(s) 5 toform the Mg_(1−x−y)Cd_(x)Zn_(y)O layer(s) 9 thereon. As previouslydescribed, the flows of gases can be produced and controlled by some orall of the injection tubes 24, 25, 26, 42, 48, dissipation plate 47,dissipation head 49, pressure flow controllers 27, 28, 29, 52, bubblers30, 31, 32, 53, containing respective liquids 33, 34, 35, 54, mass flowcontrollers 36, 37, 38, 43, 45, 52, 55, and tanks 46, 39, 51, 56containing respective gases. In step S6, during the process of growingthe Mg_(1−x−y)Cd_(x)Zn_(y)O layer(s) 9, the gases are exhausted from thechamber 2 at a rate to maintain a target pressure of gases in thechamber 2. The target pressure can be set by a user via the exhaust pump58 directly, or via a pump controller 59 which controls the exhaust pump58 based on the pressure sensed by butterfly valve 60. In step S7 adetermination is made to establish whether the growth of theMg_(1−x−y)Cd_(x)Zn_(y)O layer(s) 9 is complete. This determination canbe made using a timer (not shown) internal to the flow controller 41 ormonitored by a user to establish whether a predetermined period of timehas elapsed from start of growth of the layer(s) 9. By knowing the layergrowth rate under the process parameters (e.g., reactant species,reactant mass flow and concentration, chamber pressure, substratetemperature, substrate spin rate, and reaction time) and the desiredthickness of the substrate, the user can determine the amount of timenecessary for the reaction to continue in order to grow the layer(s) 9with the desired thickness. The present thickness standard forhomogeneous p-type layer(s) 9 is at present one-hundred (100) micronsfor integration of electronic devices, but the layer thickness standardtends to be reduced over time as integrated device features becomesmaller due to improving integration technology. Alternatively, forelectro-optic and opto-electric devices, because ZnO is inherently ann-type conductivity material, it is possible to make alternating p-typeand n-type layers of a few nanometers to several nanometers thickness toproduce an active layer stack for use as a LD or optical sensor, forexample. This can be done merely by modifying the flow of p-type dopantgas to a level sufficiently high to produce a p-type layer of desiredconductivity, and sufficiently low to produce a n-type layer of desiredconductivity. If the determination in step S7 is negative, e.g.,insufficient time has elapsed for the gases to form the layer(s) 9 tothe desired thickness, then the process continues by repeat of steps S3through and subsequent steps. The repeat of steps S3 through S8 can beperformed with process parameters that remain static throughout theprocess. Alternatively, steps S3 through S8 can be performed usingupdated process parameters corresponding to the elapsed time from startof the process. Returning to consideration of step S7, if it isdetermined that growth of layer(s) 9 on substrate(s) 5 is complete,e.g., enough time has elapsed for the layer(s) to grow the desiredthickness given the growth rate, in step S8 the flow of zinc-containingreaction gas, and optionally also magnesium- and cadmium-containingreaction gases, as well as the dopant gas, is halted. In step S9, thesubstrate(s) 5 and layer(s) 9 are permitted to cool to room temperatureby cutting off electric current to the heater 13 using the temperaturecontroller 14. In step S10 the flow of oxygen and carrier gases isstopped. This can be done after the substrate(s) 5 and layer(s) 9 havecooled to room temperature. In step S11 the rotation of the substrate isstopped. In step S12 the chamber is evacuated using the exhaust pump 58.In step S13 the substrate(s) 5 and the Mg_(1−x−y)Cd_(x)Zn_(y)O layer(s)9 thereon are extracted from the chamber 2. The Mg_(1−x−y)Cd_(x)Zn_(y)Olayer(s) 9 of the substrate(s) 5 can then be used to form electrical,electro-optic, or opto-electric devices. One or more of steps S1 throughS13 can be performed under control of the main controller 61 byactivating and deactivating the controllers 10, 14, 41, 59 in accordancewith the process parameters set for layer growth. The present inventionwill be described below by way of Examples which follow. In thefollowing Examples, the apparatus of FIG. 1 was used to grow a ZnO-basedII-VI Group compound semiconductor crystal layer on a zinc oxidesubstrate.

EXAMPLE 1

A p-type ZnO layer was grown on a ZnO substrate by the following steps.

1. Chemically cleaned, n-type ZnO single crystal substrates 5 of (002)crystallographic orientation were placed on the substrate carrier plate3. This was then loaded onto the reactor susceptor 4 through port 6 andthe chamber 2 sealed.

2. The chamber 2 was evacuated using the exhaust pump 58 to less thanone-tenth (0.1) torr pressure.

3. A flow of three-thousand (3000) sccm argon and three-hundred (300)sccm oxygen was introduced into the chamber 2 through injection tube 42via controllers 41, 43, 45, and tanks 44, 46 and the chamber pressureregulated to ten (10) torr with pump 58.

4. Meanwhile, the substrate susceptor temperature was increased tofour-hundred (400) degree C. and rotation rate increased to six-hundred(600) revolutions per minute (rpm) in a period of sixty (60) minutes.

5. This state was maintained a period of time until the temperature ofthe susceptor stabilized at four-hundred (400) degrees C.

6. Diethylzinc vapor was introduced into the chamber at a rate of1.4×10⁻⁴ moles per minute (mol/min) entrained in an argon carrier flowof three-hundred-fifty (350) sccm using controllers 27, 36, 41, bubbler30, and zinc-containing liquid 33.

7. Meanwhile, tertiarybutylphosphine 54 was introduced into the chamberat a rate of 1.5×10⁻⁵ mol/min entrained in an argon flow ofthree-hundred-fifty (350) sccm using controllers 41, 52, 55, bubbler 53,and tank 56 of argon gas.

8. Meanwhile, the previous three-thousand (3000) sccm argon andthree-hundred (300) sccm oxygen flows mentioned were maintained, and thechamber pressure was regulated at ten (10) torr.

9. This state was maintained for one (1) hour during growth of the films9 on substrates 5.

10. Subsequently, all gas flows were halted except for one-thousand(1000) sccm argon carrier gas from tank 46 via controllers 41, 45, andthree-hundred (300) sccm oxygen via controllers 41, 43 and tank 44. Thechamber pressure was maintained at ten (10) torr.

11. The state of the system was maintained for sixty (60) minutes afterdeactivating the heater 13 with the temperature controller 14 while thesubstrates 5 cooled to near room temperature.

12. All gas flows were halted, and the chamber 2 was evacuated using theexhaust pump 58 to less than one-tenth (0.1) torr.

13. The chamber was then vented with atmosphere, and the substrates wereremoved.

EXAMPLE 2

MgZnO alloy films were grown on ZnO substrates by the following steps.

-   -   1. Chemically cleaned, n-type ZnO single crystal substrates 5        of (002) crystallographic orientation were placed on the        substrate carrier plate 3. This was then loaded onto the reactor        susceptor 4 and the chamber 2 sealed.    -   2. The chamber 2 was evacuated using the exhaust pump 58 to less        than one-tenth (0.1) torr pressure.3. A flow of        three-thousand (3000) sccm argon and three-hundred (300) sccm        oxygen was introduced into the chamber 2 through injection tube        42 via controllers 41, 43, 45, and tanks 44, 46 and the chamber        pressure regulated to ten (10) torr with pump 58.4. Meanwhile,        the substrate susceptor temperature was increased to        four-hundred (400) degree C. using heater 13, temperature sensor        15 or 19, and temperature controller 14, and rotation rate        increased to six-hundred (600) rpm in a period of sixty (60)        minutes. This state was maintained a period of time until the        temperature of the susceptor 4 stabilized at four-hundred (400)        degree C.    -   3. Diethylzinc vapor was introduced into the chamber at a rate        of 2.9×10⁻⁵ mol/min entrained in an argon carrier flow of        three-hundred-fifty (350) sccm using controllers 27, 36, 41,        bubbler 30, and zinc-containing liquid 33.    -   4. Meanwhile, bis(methylcyclopentadienyl) magnesium vapor was        introduced into the chamber 2 via controllers 29, 38, 41,        bubbler 32, magnesium-containing liquid 35, and tank 39 of argon        carrier gas 41, at a rate of 5.8×10⁻⁶ mol/min entrained in an        argon carrier flow of three-hundred-fifty (350) sccm.    -   5. Meanwhile, the previous three-thousand (3000) sccm argon and        three-hundred (300) sccm oxygen flows mentioned were maintained        with units 42-47, and the chamber pressure was regulated at        ten (10) torr with exhaust pump 58.    -   6. This state was maintained for ninety (90) minutes during        growth of the films.    -   7. Subsequently, all gas flows were halted except for        one-thousand (1000) sccm argon carrier gas from tank 46 via        controllers 41, 45 and three-hundred (300) sccm oxygen via        controllers 41, 43 and tank 44. The chamber pressure was        maintained at ten (10) torr by exhaust pump 58.    -   8. The state of the system was maintained for sixty (60) minutes        after deactivating the heater 13 using the temperature        controller 14 while the substrates cooled to near room        temperature.    -   9. All gas flows were halted, and the chamber 2 was evacuated        using the exhaust pump 58 to less than one-tenth (0.1) torr.    -   10. The chamber 2 was then vented with atmosphere, and the        substrates 5 were removed through port 6.

Due to the fact that the substrate and Mg_(1−x−y)Cd_(x)Zn_(y)O layergrown thereon are relatively pure and are closely matched in theircrystalline lattice spacing, as well as the activation of a large numberof dopant atoms in the layer, net acceptor concentration within thelayer is relatively high and resistivity is relatively low as comparedto layers produced with previous methods. For example, it is possible toproduce layers with net acceptor concentrations of at least 10¹⁷/cm³ andresistivities of at least one-tenth (0.1) Ohm-cm. Hence, the methodsdisclosed herein are useful for generating relatively high-performance,commercially-viable devices such as LEDs, LDs, and FETs.

Although the methods of the invention have been described herein withreference to specific embodiments and examples, it is not necessarilyintended to limit the scope of the invention to the specific embodimentsand examples disclosed. Thus, in addition to claiming the subject matterliterally defined in the appended claims, all modifications,alterations, and equivalents to which the applicant is entitled by law,are herein expressly reserved by the following claims.

1. A method comprising: forming a p-type II-VI Group compoundsemiconductor represented by the formula Mg_(1−x−y)Cd_(x)Zn_(y)O, inwhich 0≦x<1, 0<y≦1, and x+y=0.1 to 1, on a zinc oxide (ZnO) substrate.2. The method of claim 1 wherein y=1 and x=0.
 3. The method of claim 1wherein the ZnO substrate has an (002) crystallographic orientation. 4.The method of claim 1 wherein the forming comprises: heating thesubstrate; and supplying reaction gases comprising a first gascontaining a zinc compound, a second gas containing oxygen, a thirdp-type dopant gas with at least one element from one of Groups IA, IB,VA and VB of the periodic table of the elements, and a fourth inertcarrier gas used to carry the reaction gases to the surface of theheated substrate to grow the p-type II-VI Group compound semiconductorcrystal layer on the substrate.
 5. A method as claimed in claim 4wherein at least one of the gases is directed in a flow transverse tothe substrate to carry the reaction gases to the substrate to form thelayer.
 6. A method as claimed in claim 4 wherein the heating andsupplying is performed in a chamber.
 7. A method as claimed in claim 4wherein the temperature of the substrate is maintained at a temperaturein a range from two-hundred-fifty (250) to six-hundred-fifty (650)degrees Celsius during growth of the layer.
 8. The method of claim 4wherein the zinc compound in the first gas comprises diethylzinc,dimethylzinc, or a mixture thereof.
 9. The method of claim 4 whereinsaid first gaseous material further comprises magnesium metalorganic,cadmium metalorganic or a mixture thereof.
 10. The method of claim 4wherein the oxygen in the second gas comprises oxygen (O₂) or nitrousoxide (N₂O).
 11. The method of claim 4 wherein the third p-type dopantgas comprises at least one of nitrogen (N), copper (Cu), arsenic (As),and phosphorus (P).
 12. The method of claim 4 wherein the third p-typedopant gas is selected from the group consisting ofbis(tetramethylheptanedianol) copper, arsine, phosphine, ortertiarybutylphosphine.
 13. The method according to claim 4 wherein saidinert carrier gas comprises argon gas.
 14. The method of claim 4 whereinthe gas flow rates are maintained at from ten (10) to five-thousand(5000) standard cubic centimeters per minute (sccm) during growth of thelayer.
 15. The method of claim 4 wherein the gas flow rates aremaintained by mass flow controllers.
 16. The method according to claim 4wherein the gases are maintained under a pressure in a range from five(5.0) to fifty (50.0) torrs during growth of the layer.
 17. The methodof claim 16 wherein the gases are maintained under pressure by anexhaust pump which controls the rate of exit of the gases from a chamberin which the substrate is situated during growth of the layer.
 18. Themethod of claim 18 wherein the pressure of the gases is furthermaintained by pressure flow meters.
 19. The method of claim 4 whereinthe substrate is rotated at a rate in a range from one-hundred (100) toone-thousand (1,000) revolutions per minute (rpm) during growth of thelayer.
 20. A method as claimed in claim 4 wherein the temperature of theZnO substrate is maintained at a temperature in a range fromtwo-hundred-fifty (250) to six-hundred-fifty (650) degrees Celsiusduring growth of the layer.
 21. A method as claimed in claim 4 whereinthe heating is performed by a heater comprising anelectrically-resistive element.
 22. A method as claimed in claim 4wherein the substrate is maintained at a target temperature by atemperature controller and temperature sensor.
 23. The method of claim 1wherein the substrate has an (002) crystallographic orientation.
 24. Themethod of claim 1 wherein the ZnO substrate is produced by an apparatusthat contains liquid-phase ZnO in a solid-phase ZnO “skull” duringgrowth of the crystal from which the ZnO substrate is formed.
 25. Themethod of claim 1 wherein the substrate is selected from the groupconsisting of zinc oxide (ZnO), sapphire (Al₂O₃), silicon carbide (SiC),silicon (Si), gallium arsenide (GaAs), and gallium nitride (GaN). 26.The method of claim 1 wherein the substrate is rotated by a drive unitconnected to a shaft that turns a susceptor and carrier plate upon whichthe substrate is situated during growth of the layer.
 27. The method ofclaim 1 wherein the acceptor concentration of the zinc-oxide-based II-VIGroup compound semiconductor crystal layer produced by the method is atleast 10¹⁷ atoms per cubic centimeter.
 28. The method of claim 1 whereinthe resistivity of the zinc-oxide-based II-VI Group compoundsemiconductor crystal layer produced by the method is at least one-tenth(0.1) Ohm-centimeter.